Save every other pixel of an image at half res
In your case where you are looking for exactly ½ resolution there is a very simple solution, just use the scale node in the compositor.  

This is a 20x20px image; enlarged 16 times (to show detail). Here is the original if you want to try it.


Now after going through the scale node, you can see that every other pixel from the original is in the scaled image (again scaled up 16 times).


All you have to do is set up the compositor nodes: Image, Scale, and Composite nodes.




However if you need every other pixel starting with the second row or column of pixels you can add in a translate node to move the image before you scale it. Then after the scale node move the image back to the center.

This node setup will give you every other pixel starting with the second column. (Use the Y axis for the rows).




Last set the render size to ½ the resolution of the input image. So for a 10 megapixel image you could type 3872/2 in the resolution X field. In the render tab of the properties window.